A bug. It's fixed now on the trunk and release branches. Thanks.
Thank you, Eric. That works, but now proto::lazy isn't working with proto::fold. Below is a program you posted to boost.devel on 3/29/2008, updated with s/bind/lazy/ and s/_arg/_child/. It's giving me the following syntax error. Any idea what's wrong? Thanks, Dave Jenkins error C2664: 'Accumulator<IsValid,Value>::Accumulator(const Accumulator<IsValid,Value> &)' : cannot convert parameter 1 from 'Accumulator<IsValid,Value>' to 'const Accumulator<IsValid,Value> &' c:\boost\boost\proto\transform\call.hpp 156 #include <iostream> #include <boost/assert.hpp> #include <boost/mpl/or.hpp> #include <boost/mpl/and.hpp> #include <boost/type_traits/is_same.hpp> #include <boost/type_traits/is_void.hpp> #include <boost/proto/proto.hpp> #include <boost/proto/transform.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; template<typename IsValid = mpl::true_, typename Value = void> struct Accumulator : IsValid { template<typename Value2> struct validate : mpl::and_< IsValid , mpl::or_< boost::is_void<Value> , boost::is_same<Value, Value2> > > {}; template<typename Sig> struct result; template<typename This, typename Value2> struct result<This(Value2)> { typedef Accumulator<validate<Value2>, Value2> type; }; template<typename Value2> typename result<void(Value2)>::type operator ()(Value2 const &) const { return typename result<void(Value2)>::type(); } }; struct SameTerminals : proto::or_< proto::when< proto::terminal<proto::_> // treat the current state as a callable // object and pass it the value of the terminal. , proto::lazy<proto::_state(proto::_child)> > , proto::otherwise< proto::fold<proto::_, proto::_state, SameTerminals> > > {}; int main() { int dummy = 0; proto::terminal<int>::type i = {0}, j = {1}, k = {2}; proto::terminal<short>::type s = {42}; BOOST_ASSERT( SameTerminals()(i+j*k, Accumulator<>(), dummy) ); BOOST_ASSERT(!SameTerminals()(i+s*k, Accumulator<>(), dummy) ); }